Module 12: Glossary




Absorption: The process by which radiation transfers some, or all, of its energy to the medium through which it is passing.

Absorbed Dose: Absorbed dose is a measure of the energy deposited by radiation in a certain material. The SI unit for absorbed dose is the gray (Gy). One gray is an absorbed radiation dose of 1 joule per kilogram. 1 Gy = 1 J/kg.

Absorption Coefficient, Linear (m): The fractional decrease in the intensity of a beam of gamma or X-radiation as it passes through an absorbing medium. It is expressed per unit thickness of medium (usually cm-1). The specific value of the linear absorption coefficient depends on the type of absorbing material, the energy of the gamma or X-ray and the density of the absorbing medium. The value of m is used in the equation I = IoAe-mx  I = IoAe-mx; where I is the intensity and x is the thickness of the absorber in the same base units as m.

Absorption Coefficient, Mass (cm2/g): The linear absorption coefficient of a material divided by the density of the absorbing material. It is expressed in units of cm2/g. The mass absorption coefficient is independent of the density or nature of the medium. For example, the mass absorption coefficient of water for 1.0 MeV gamma rays is 0.0707 cm2/g. The linear absorption coefficient of water as liquid (20°C) is (0.0707 x 0.998234) = 0.0706 cm-1; and water as ice (-20°C) is (0.0707 x 0.99349) = 0.0702 cm-1

Activation: The process of making a material radioactive by the absorption of neutrons, protons, photons, etc in a material. Activation is used in a nuclear reactor to allow the analysis of very small amounts of material by means of the radiation given off during the decay process.

Activity: The rate of nuclear transformations or transitions occurring in a radioactive source. The SI unit of activity is the becquerel (Bq), which equals one disintegration per second.

Acute Effects: These effects usually appear shortly after exposure to high levels of radiation. Among these effects are inflammation of the skin, nausea and vomiting, blood changes, bone marrow damage, etc.

Alpha Particle (α): A charged particle consisting of two neutrons and two protons. It is sometimes emitted from the nucleus of high atomic number elements during radioactive decay. The alpha particle is the nucleus of a Helium-4 atom. Due to the +2 charge, it causes ionizations along its path length. An alpha particle will travel a few centimeters in air and is stopped by a sheet of paper or the skin’s layer of dead cells.

Annihilation Radiation: The radiation emitted as a result of the interaction of a positron (positively charged beta particle) and an electron (negatively charged beta particle). The annihilation of these particles results in the disappearance of the two particles and the formation of two gamma rays, each of 0.511 MeV, which are oppositely directed.

Annual Limit on Intake (ALI): An ALI is the amount of activity of a radioisotope, measured in becquerel, which if ingested or inhaled while working, will result in a maximum committed dose of 20 mSv in a “reference person” for the next 50 years.

Anti-neutrino, Electron: Elementary particle without rest mass which accompanies beta minus (electron) particles during disintegration. The electron anti-neutrino shares energy with the electron, which is why the beta energy spectrum is continuous.

As Low As Reasonably Achievable (ALARA): Is the principle of radiation protection demanding that the number of people exposed, and the magnitude of their individual doses should be kept as low as reasonably achievable (ALARA), taking into account economic and social factors (optimization of protection)

Atom: Greek origin (atoms = indivisible), the atoms are the fundamental building blocks of substances. Contrary to their names, atoms have structure. The heavy nucleus is surrounded by lighter electrons moving around in shells

Atomic Number (Z): The number of protons in the nucleus of an atom. The atomic number determines the chemical properties of the element. Atoms with the same atomic number but with different numbers of neutrons are called isotopes.

Atomic Mass Number (A): The total number of neutrons and protons in the nucleus of an atom.

Attenuation: The reduction in the intensity of radiation as a result of being absorbed in a medium as it passes through the medium.


Background Radiation: Radiation arising from natural sources in the environment. The main sources of ionizing radiation in the environment are: from radon gas in the air, radiation from rocks and soil, radiation from radioactive materials in the body and cosmic radiation. The annual dose equivalent from natural background radiation sources in Toronto, Ontario is approximately 2 mSv.

Becquerel (Bq): The SI unit of radioactivity. It is equivalent to 1 disintegration per second.

Beta Particle (β): A charged particle emitted during a beta disintegration. It has a mass equal to 1/1837 of the mass of the proton and an electrical charge equal in value. When it’s electrical charge is negative, it is called an electron. When the electrical charge is positive, it is called a positron.

Bioassay: The assessment of the uptake of radioactive materials into the body. Two methods are available, in vitro and in vivo. The former involves taking a specimen, usually a fluid such as urine, and measuring the radioactivity in it by use of a suitable counter. The material is assessed externally to the body. In vivo techniques involve placing a radiation monitor near the body and measuring the radioactivity being emitted from radioactive material within the body. An example of this is the use of a detector placed near the thyroid to measure uptake of I-125 by counting the gamma radiation emanating directly from the gland.

Bremsstrahlung: A phrase derived from the German language, it literally means “braking radiation”. When a charged particle (generally referring to beta particles) passes close to a nucleus, a change in the velocity of the particle will cause a loss of particle energy by electromagnetic radiation. The bremsstrahlung photons have a continuous spectrum of energy distribution below the maximum kinetic energy of the charged particle. The likelihood of bremsstrahlung production increases with the atomic number of the absorbing material. Therefore, only low Z materials are used for protection against beta radiation.


Calibration: The process of determining the efficiency of equipment used for radiation monitoring. The response of the instrument to a source of known activity is measured. The resulting efficiency is used to determine unknown activities.

Carcinogenic: The ability of a material to cause cancer, whether it is chemical, biological or physical in nature. b-naphthylamine is an example of a carcinogenic chemical and ionizing radiation is an example of a carcinogenic physical agent.

CNSC: Acronym for the Canadian Nuclear Safety Commission, the federal agency regulating the possession and use of radioactive materials in Canada.

Committed Dose: The total equivalent and effective radiation dose received from a radioactive substance in the body during the 50 years following the intake of that substance for adults, or to the age of 70 for children.

Contamination (Radioactive): Radioactive material deposited on a surface or in a medium where it is not wanted. Surface contamination is monitored directly with portable instruments or indirectly through the use of the swipe test.

Counter, Scintillation (also Liquid Scintillation Counter or LSC): An instrument designed to measure very small amounts of radioactivity, generally from negative beta decay. It involves placing the radioactive material in a vial containing a scintillation fluid. As the radiation is given off, it interacts with the fluid and causes excitation of the molecules. Organic compounds in the mixture convert the excitation energy to fluorescence. The light emitted during the fluorescence is detected by photomultiplier tubes positioned outside of the vial. The signal from the photomultiplier tubes is processed and then converted into counts per minute. Due to the fact that the radioactive material is intimately mixed with the primary detector (the scintillation fluid), detection efficiency for low energy beta emitters is much higher than with other means of detection.

Curie (Ci): The former unit for expressing activity. The curie was originally based on the decay of 1 gram of radium and is equivalent to 37 billion disintegrations per second. More common units are the millicurie (mCi) and the microcurie (mCi). This unit is being replaced by the SI unit known as the becquerel (Bq). One mCi is equivalent to 37 MBq.


Daughter nucleus (Progeny): The resulting nucleus of the radioactive decay. Sometimes an unstable nucleus, itself a daughter nucleus, can suffer radioactive decay becoming a parent nucleus for this new radioactive process. In these cases, parent and daughter nuclei form a radioactive decay series.

Decay Constant (λ): The fraction of the number of atoms of a radioisotope that decay in a unit time. It is expressed as the reciprocal of time (e.g. seconds-1) and is related to the half-life by the following equation: Decay Constant

Decay, Radioactive: The nuclear transformation of a parent nucleus resulting in a progeny nucleus. During this process, the emission of particles and/or electromagnetic energy can occur. This is also called a disintegration.

Decision Limit: A statistical tool applied to identify whether a radiation measurement is within the background or not. It is also used to calculate the minimum detectable activity (MDA) for the method/instrument used to do the measurement.

Decommissioning: Actions taken in the interest of health, safety, security and protection of the environment, to retire a nuclear facility permanently from service. May also be used to refer to the cleaning of a radioisotope laboratory, equipment, furniture, etc., so that it may be removed from a radioisotope permit.

Delay and Decay: The storage of radioactive waste containing radionuclides with short half-lives for a sufficient time to enable their unrestricted discharge to the environment when their final activity level is below regulatory limits.

Delayed Effects: These effects appear much later after exposure. They can arise from repeated exposures to relatively low levels of radiation, or from a single exposure to higher levels of radiation. For example different forms of cancer, life-shortening, cataracts, genetic effects, etc.

Deterministic Effects: See Non-stochastic Effects.

De-excitation: The process by which an excited system releases energy to achieve a more stable state.

Disintegration: See Decay, Radioactive.

Disposal, Waste: The permanent and secure containment of radioactive wastes, with no intention to retrieve them.

Dose, Absorbed: (see Absorbed Dose)

Dose, Artificial: The dose received by a person from artificial sources.  In Canada, the CNSC imposes a limit of 1mSv/yr for the public from artificial sources other than for medical purposes. The main contribution to artificial dose, for average Canadian, is medical exposure.

Dose, Effective: The tissue weighted sum of the equivalent doses in all specified tissues and organs of the body, given by expressions: E = ∑T wTR wR DT,R or E = ∑T wT HT where HT is the equivalent dose in a tissue or organ, and WT is the tissue weighting factor. The unit for the effective dose is the same as for the unit for absorbed dose, J/kg, and its special name is sievert (Sv)

Dose, Equivalent: The dose in a tissue or organ T given by: HT = ∑R wR DT,R, where DT,R is the mean absorbed dose from radiation type R (alpha, beta, gamma, neutrons, etc.) in a tissue or organ T, and WR is the radiation weighting factor. Since WR is dimensionless, the unit for equivalent dose is the same as the unit for absorbed dose, J/kg, and its special name is sievert (Sv)

Dose, External: The dose received by a person when the radiation source is situated outside his/her body. TLDs are used to measure the external dose. Protection against external exposure is achieved by applying the principle of time, distance and shielding.

Dose, Internal: The dose received by a person from a radiation source that has entered the body. Bioassays are used to measure internal exposure. Inhalation and ingestion of radioactive materials can be prevented by following laboratory rules and good work practices.

Dose Limits: Is the principle of radiation protection demanding that the total dose to any individual from regulated sources in planned exposures situation, other than medical exposure of patients, should not exceed the appropriate limits established by the government.

Dose, Natural: The dose received by a person from natural sources. Natural irradiation is both internal and external. The natural dose from background radiation in Canada is around 2 mSv/yr. The natural dose can vary by a factor of more than 10.

Dose Rate: Dose received by an individual per unit of time, J/(kg*s). When referring to the effective dose, the unit of measure can be Sv/hr, mSv/day, etc.

Dosimeter: A device used to measure and record the dose of radiation to which a person has been exposed. There are whole body, extremity, skin dosimeters, etc.

Dosimetry: The process of finding the radiation dose. Carried out by either practical measurements or theoretical evaluation.

DPM: Acronym for disintegrations per minute.


Efficiency (Counter): The ratio between the number of counts registered by the instrument and the number of disintegrations of the radioactive source. Efficiency is usually expressed as a percentage. It is a function of the geometry and design of the detector, as well as the internal electronics. It is also dependent on the type and energy of radiation being monitored.

Electron Capture (EC), Orbital: An unstable proton-rich nucleus may capture an orbital electron (as an alternative to beta plus disintegration) to solve the instability. The electron from the inner shell will react with a proton, changing the nucleus into one with a lower Z number (decreased by 1) in an excited state. During the process, a neutrino is also emitted. From the rearrangement of electrons in the new atom, x-rays will be emitted. The excited progeny nucleus can get rid of the energy by gamma emission or by internal conversion.

Electron Volt (eV): A measure of energy. It is commonly used for expressing the energy of particles and/or electromagnetic radiation emitted during radioactive decay. It is the amount of energy gained by an electron travelling through a potential difference of one volt. Common multiples include the kilo electron volt (keV) and the mega-electron volt (MeV).

Element, Chemical: All atoms belonging to a chemical element have the same atomic number Z (the same number of protons) and the same number of electrons. They all have the same chemical properties and occupy the same position in the periodic table of elements. All atoms forming a chemical element are called isotopes of that element. There are more than 105 different chemical elements.

Equivalent Dose: See Dose, Equivalent

Erythema: Reddening of the skin which can be caused by exposure to radiation. The skin erythema dose (SED) was a unit of radiation exposure in the early part of the 1900’s. It is due to the dilation of the capillaries in the skin and occurs with exposure to ionizing radiation doses of about 10 Sv to the skin.

Excitation: The unstable state of an atom or a nucleus caused by external agents or by radioactive decay. When caused by external agents, the system (atom or nucleus) absorbs energy from the surroundings. An excited system, sooner or later, undergoes the process called de-excitation.

Exposure Dose: The measure of ionization produced in air by gamma or X-radiation. Originally measured in Röntgens (R), the current SI equivalent is coulomb per kilogram of air. 1C/kg = 3876 R.

Exposure, Acute: Exposure to a high dose during a short period of time which may produce biological effects within a short period after exposure.

Exposure, Chronic: Continuous exposure to radiation for long periods of time which may cause delayed effects.


Fission Reaction: The nuclear reaction resulting in the splitting of a heavy nucleus into two or more nuclei. Accompanied by particles (usually 2 or 3 neutrons) and gamma-ray emissions. The resulting nuclei and accompanying radiation carry large amounts of energy (approximately 200 MeV at the fission of a U-235 nucleus). It can be spontaneous or provoked by neutron absorption. Under certain conditions, neutron production and consumption can sustain the reaction (chain fission reaction). It is used in energy production (in nuclear reactors) or for military purposes (atomic bomb).

Fusion Reaction: The nuclear reaction resulting in the unification of two light nuclei to obtain a new nucleus. Accompanied by particles and energy emission. It is the reaction that produces energy in stars. Still at the experimental stage for energy production, it is used for military applications (so-called “hydrogen bomb”).


Gamma Ray (γ): An energetic photon emitted from the nucleus during radioactive decay. The energy spectrum of gamma rays is discrete. Protection from gamma radiation requires lead or concrete shielding. Gamma radiation usually accompanies other types of decay.

Geiger Müller Tube (G-M tube): The main component of most commonly available radiation detection instruments. It consists of a hollow tube filled with a gas and contains a central electrode running parallel to the length of the tube. The shell of the tube forms the other electrode. The tube is held at a high potential voltage, approximately 800-1200 volts and radiation passing through the gas will cause it to become ionized. The ionization is amplified and detected by the supporting circuitry. The G-M tube may also have a small amount of material wrapped around it to improve its response over a wide range of radiation energies and is known as an energy compensated detector. If the end of the tube is made of a thin material such as mylar, it is called a thin end window detector and the G-M tube can be more sensitive to some alpha and beta radiation.

Genetic Damage: Damage caused to genes in cells that are part of the reproductive organs. Genetic damage does not affect the current generation but may be passed on to future generations.Gray (Gy). The SI unit of absorbed dose is equivalent to one joule per kilogram. Used to measure deterministics (organ or tissue) effects of radiation.


Half-life, Physical: The characteristic time taken for the activity of a particular radioactive material to decay to half of its original value; that is, for half the atoms initially present to disintegrate.

Half-life, Biological: The characteristic time required for the amount of a substance to be reduced to one-half of its initial value from metabolism alone. The biological half-life of a radionuclide does not depend on the radioisotope but on the organ or body system in which the material is deposited, as well as the chemical properties of the radioactive material.

Half-life, Effective: The characteristic time required for a radioactive material to be eliminated from a biological system through a combination of the physical and biological removal processes. The effective half-life is a mathematical combination of the physical and biological half-lives of the particular radioisotope.

Half-Value Thickness (HVT): The thickness of shielding material required to reduce the intensity of a given type of radiation to one-half of the original amount. Related to the tenth value thickness (TVT).

Health Physics: The branch of science dealing with radiation protection. It arose as a result of the development of the atomic bomb in the Manhattan Project. There is some suggestion that the phrase arose as a result of the need for the secrecy surrounding the development of the bomb. Supposedly, words associated with radiation could not be used and so it was decided to call the field health physics. Persons working in the field of radiation protection may also be referred to as health physicists.


IAEA: Acronym for the International Atomic Energy Agency. It is an international body within the United Nations that provides advice and assistance to the member nations on the use of radioactive materials.

ICRP: Acronym for the International Commission on Radiological Protection. Originally known as the International X-ray and Radium Protection Committee founded in 1928, it was reorganized in 1950 to become the ICRP. It is composed of a Chair and not more than 12 members chosen on the basis of their expertise in specific areas without regard to nationality. The ICRP publishes recommendations on radiation protection that are usually the basis of legislation for radiation protection.

Inverse Square Law: The relationship between distance and intensity for gamma and X-radiation. The intensity from a point source is inversely proportional to the square of the distance from the source.

Internal Conversion (IC): An excited nucleus transmits its energy to an electron from an inner electronic shell (usually K or L). The electron is ejected from the atom carrying the de-excitation energy. The IC electrons are mono-energetic, therefore, they differ from beta minus electrons by both origin and energy spectrum. The IC process is always accompanied by X-ray emissions. IC is an alternative mechanism for gamma emission.

Ion: An atomic particle, atom or chemical radical that carries a net electrical charge, either positive or negative.

Ionization: The process by which electrons are removed or added to atoms to create ions. Radiation that possesses enough energy to remove orbital electrons is called ionizing radiation.

Ionization Chamber: A chamber used for the measurement of radiation exposure. Similar to a Geiger Müller tube, it is operated at much lower electrical potentials. The fundamental principle of gas ionization by radiation still applies but since the potential voltage is not as great, the amount of amplification in the tube is small. Generally used for personal dosimeters and standardization instruments.

Irradiation: The exposure of a material to radiation.

Isotopes: Atoms with the same atomic number (number of protons in the nucleus) but having a different atomic mass number (because of different numbers of neutrons in the nucleus). Chemically, isotopes of a given element all behave the same although some may be radioactive.


Justification: Is the principle of radiation protection demanding that any decision that alters the radiation exposure should do more good than harm.


Labelled Compound, Radioactive: A molecule that has had one of its atoms replaced by a radioactive isotope. Once labelled, the path of the molecule can be traced through a biological system.

Latent Period: The period of time between the exposure to radiation and the expression of radiation injury. Generally applied to cancer induction from chronic radiation exposure, the latent period can be anywhere from 5-10 years for leukemia, to 20-30 years for other types of cancers.

Linear Energy Transfer (LET): A measure of the rate at which an energetic particle transfers energy to the surrounding medium. Alpha particles have a high LET, while beta particles have a lower LET. The unit for LET is J/m or keV/µm.

Linear-non-threshold (LNT) model: A dose-response model which is based on the assumption that, in the low dose range, radiation doses greater than zero will increase the risk of excess cancers and/or heritable disease in a simple proportional manner.


Minimum Detectable Activity (MDA): MDA is a characteristic of the instrument or measurement method. It indicates the instrument’s (or method) limitation to detect radioactive material.


Neutron: A nuclear particle having a mass similar to a proton but having no electrical charge. During negative beta decay  (β), a neutron disintegrates into a proton an electron and an anti-neutrino which are then ejected from the nucleus. Neutrons can exist outside of the nucleus and have a high potential for radiation damage since they lose energy in biological materials through scattering. Shielding for neutron sources involves using materials containing large amounts of hydrogen.

Neutron Emission: Accompanies a fission reaction or other type of nuclear reaction. Depending on the type of reaction, the energy spectrum can be discrete or continuous.

Neutrino: Elementary particle without rest mass which accompanies beta plus (positron) particles during disintegration. The neutrino shares energy with the positron giving a continuous beta energy spectrum. It is also emitted during electron capture.

Non-Stochastic Effects (Deterministic Effects): Injury in a population of cells, characterized by a threshold dose and an increase in the severity of the reaction as the dose is increased. Also termed tissue or organ reaction. An example of a non-stochastic effect is cataract formation in the lens of the eye.

Nuclear Energy Worker (NEW): A person who, during the course of occupational work, is likely to receive a dose of ionizing radiation in excess of the exposure limit allowed for members of the general public.

Nuclear Reaction: A reaction involving one or more nuclei resulting in the creation of one or more new nuclei. Usually accompanied by particles (electrons, protons, neutrons, alpha, etc.) and /or gamma-ray emissions.

Nuclide: A general term referring to all isotopes of an element.


Optimization of Protection: Is the principle of radiation protection demanding that the number of people exposed, and the magnitude of their individual doses should be kept as low as reasonably achievable (ALARA), taking into account economic and social factors.


Parent Nucleus: The original nucleus in a radioactive decay.

Permit Holder: Person who is issued a permit under the university’s licence. It must be approved by UTRPA. Permit Holders are held responsible at all times for all aspects of radiation safety in areas under their supervision.

Photon: A quantum of energy emitted in the form of electromagnetic energy. Gamma photons originate in the nucleus and X-ray photons originate in electronic shells and from the bremsstrahlung process.

Point Source: A source of radiation, the physical size of which is small by comparison with the distance at which the radiation is monitored. It can be considered to arise from a single point.

Positron (β+): A positively charged electron, emitted from the nucleus during some forms of radioactive decay. A positron will combine with an electron (b) and result in the production of annihilation radiation. See also Beta Particle.

Principles of Radiation Protection: The current philosophy in radiation protection is covered by three principles: justification, optimization (ALARA) and dose limits.


Quality Factor (QF): A modifying factor used in the derivation of dose equivalent. Used to allow comparison of different types of radiation. Also known as the Radiation Weighting Factor. It varies from 1 to 20.


Radiation: The emission and propagation of particles and electromagnetic rays. Generally used to refer to ionizing radiation.

Radiation, Ionizing: Radiation which removes orbital electrons from atoms, or breaking the molecular bonds, thus creating ion pairs. Alpha and beta particles are more densely ionizing than gamma rays or X-rays of equivalent energy.

Radiation Protective Services (RPS), U of T: It is an administrative service within U of T Office of Environmental Health and Safety which carries out the daily operation of the radiation safety program, as directed by the UTRPA.

Radiation Weighting Factor: A dimensionless factor by which the organ or tissue absorbed dose is multiplied to reflect the higher biological effectiveness or high-LET radiation compared with low-LET radiation. It is used to derive the equivalent dose from the absorbed dose averaged over a tissue or organ.

Radioactive Decay Series: Two or more radioactive decay processes in which the daughter nucleus serves as a parent nucleus for the next process. There are four natural radioactive decay series and large numbers of artificial ones.

Radioactive Isotope: An unstable nuclei which undergoes a nuclear transformation.

Radioactive Material: A substance containing unstable nuclei exceeding a certain concentration limit. It is also called prescribed radioactive substance, radioactive nuclear substance or nuclear substance.

Radioactivity: The property of a certain nuclide to spontaneously emit particles or gamma or X-radiation following a nuclear transformation.

Radioisotope (Radionuclide): A radioactive isotopes.

Radioisotope User: Person using radioactive materials. He/she has specific responsibilities under different acts and regulations in order to ensure that work with radiation does not create a hazard to themselves, to others and to the environment.

Radiosensitive: Sensitive to the effects of irradiation, principally applied to biological systems. Cells of the body which are easily damaged by exposure to ionizing radiation are termed radiosensitive.

Radiotoxicity: The term referring to the potential of a radioisotope to cause damage to living tissue by the absorption of energy from the disintegration of the radioactive material that is within the body.

Reference Man: A standard model of a human being, developed by the ICRP and detailed in ICRP Report #23. The characteristics of standard man are used when specific body information is not available for dosimetry purposes.

Regulatory Dose Limit: A legal limit on radiation dose specified in the Canadian Nuclear Safety Regulations.

Röntgen Equivalent Man (rem): The older term used to describe equivalent and effective dose.  The SI unit is the Sievert (Sv). 1 Sv = 100 rem.

Röntgen, also spelled Rœntgen or Roentgen (R): Named after Wilhelm Röntgen, it is a unit of radiation exposure. Useful submultiples include the milliröntgen (mR) and the microröntgen (mR). This is gradually being replaced by the SI equivalent which is coulomb per kilogram of air.


Sealed Source: A radioactive source sealed in a container or having a bonded cover, the container or cover being strong enough to prevent contact with and dispersion of the radioactive material under the conditions of use and wear for which it is designed.

Shielding: The use of absorbing material between a source of radiation and the detector or recipient. Shielding absorbs radiation and reduces the intensity of the incident radiation. Shielding is chosen on the basis of its effectiveness for a given type of radiation, its cost and other physical attributes.

SI: Acronym for Système International, it is the international system of units of measurement.

Sievert (Sv): The SI unit for equivalent and effective dose. It is gradually replacing the rem. 1 rem = 0.01 Sv. Used to measured stochastic effects (radiation-related cancer and heritable effects).

Somatic Injury: Injury to tissues of the body other than the reproductive organs. The somatic injury affects the current generation but is not passed on to future generations.

Specific Activity (Specific Radioactivity): The activity of a radioactive material divided by its mass, volume, surface, etc.

Spectrometry (Spectroscopy): The process of identifying an unknown nucleus, atom, or substance by measuring the energy absorbed during excitation or emitted during de-excitation.

Spectrum, Energy: The distribution of the number of particles or electromagnetic rays counted by an instrument over a certain energy range. The spectrum contains the “signature” of the system (nucleus, atom or substance) and is used in spectrometry. For beta disintegration and bremsstrahlung X-rays the energy spectrum is continuous, whereas for alpha, gamma and de-excitation X-rays the spectrum is discrete.

Stochastic Effects: Malignant disease and heritable effects for which the probability of an effect occurring, but not its severity, is regarded as a function of dose without threshold.

Survey Meter: An instrument used to measure radiation, typically radiation exposure dose. The instrument usually consists of an energy compensated Geiger Müller tube and associated circuitry which causes a meter deflection or other readout in the presence of ionizing radiation.

Swipe Test: The process of measuring contamination by wiping a certain area (approx. 100 cm2) of a surface with a filter paper and placing it in a vial with scintillation fluid for counting in a scintillation counter. The efficiency of the removal of non-fixed contamination with the filter paper is considered 10%.


Tenth-value Layer: This is the amount of shielding required to reduce the intensity of gamma or X-radiation to one-tenth of its initial value.

Tissue Weighting Factor, WT: The factor by which the equivalent dose in a tissue or organ T is weighted to represent the relative contribution of that tissue or organ to the total health detriment resulting from uniform irradiation of the body. It is weighted such that: ∑T WT = 1

Thermoluminescent Dosimeter (TLD): A personal dosimeter that uses solid crystals to monitor radiation absorbed dose. Typically, these crystals are composed of lithium fluoride (LiF) and exhibit radiation absorption characteristics similar to that of human tissue. The ionizing radiation produces small local crystal defects which are stable until the crystal is heated. When the crystal is heated to temperatures of approximately 200°C, the defects are removed and the associated energy is released in the form of light. The amount of light produced is proportional to the number of crystal defects induced which in turn is related to the amount of radiation absorbed.


Uncertainty: The degree of accuracy of the measuring method and/or instrument. For radioactive measurements, the uncertainty is a summation of the uncertainty in the measurement of the sample, measurement of the background, and other possible sources of error.

University of Toronto Radiation Protection Authority (UTRPA): A committee composed by academics and administrators appointed by the U of T Governing Council to exercise a complete and all-embracing control of the radiation protection program within U of T jurisdiction.


Waste, Radioactive: Any material containing or contaminated with radionuclides in concentrations greater a certain value than would be considered acceptable for uncontrolled use or release, and for which there is no foreseen purpose.


X-ray: A form of electromagnetic energy that is produced external to the nucleus of an atom. Typically, X-rays may be produced when orbital electrons are rearranged (characteristics X-rays) or when accelerated electrons are slowed in the presence of a nucleus, such as during the production of bremsstrahlung. X-rays are similar to gamma-rays in the manner by which they are absorbed and shielded. However, whereas gamma rays have discrete energies and originate from the nucleus, X-rays can be emitted with both discrete and a broad spectrum of energies and originate outside of the nucleus.

(Back to Top)